An ignition coil is mounted to a cylinder of an internal combustion engine of a vehicle or the like. The ignition coil is electrically connected with a sparkplug to generate a spark in a combustion chamber of a cylinder. When fuel is burned in the combustion chamber, fuel is ionized, so that ion current flows between electrodes of the sparkplug. The ion current is detected to evaluate whether miss fire arises in the combustion chamber.
Ion current flowing between the electrodes of the spark plug is significantly small. Therefore, a specific condition needs to be satisfied to stably detect the ion current.
For example, as disclosed in U.S. Pat. No. 6,340,303 (JP-A-2001-85139), a sparkplug is electrically connected with a secondary coil in an ignition coil. A spring member (coil spring) is interposed between a spring supporting portion of a high voltage terminal and a head portion of the sparkplug. The spring supporting portion rises from an inner wall of the high voltage terminal, which is provided to a plug mounting hole of the ignition coil. In this structure, the spring member maintains a contact condition between the spring supporting portion and the head portion of the sparkplug, so that an instantaneous disconnection is restricted thereamong.
However, when external force such as vibration is applied to the ignition coil, the spring member vibrates between the high voltage terminal and the sparkplug. Therefore, a contact condition between the high voltage terminal and the spring member varies, and a contact condition between the sparkplug and the spring member also varies. As a result, electric resistance may change among these components.
Furthermore, an ignition coil have a following problem, regardless of whether the ignition coil includes an ion current detecting function or not. When a spring member vibrates between a high voltage terminal and a sparkplug, abrasion may be caused thereamong. As a result, abrasion powder is formed thereamong, and the abrasion power is oxidized. The oxidized abrasion power may become an electrically insulative matter. When oxidized abrasion power is interposed among the spring member, the high voltage terminal, and the sparkplug, electrical conductivity may become insufficient among the components.